Scouts are powerful and expensive units. They are tough but their expensive cost makes it relatively undesirable.
In their days of service, scouts served as point guards for the protoss fleet, being maintained in the hangars of carriers, though were often called upon to engage hostile forces. Shortly before the war against the zerg, recent developments in enhanced sensor systems and high-power engines further increased its effectiveness.
Since the end of the Brood War, the scout is steadily being phased out by the Phoenix.